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7 3URFHHGLQJVRIWKHWK0HGLWHUUDQHDQ&RQIHUHQFHRQ &RQWURO$XWRPDWLRQ-XO\$WKHQV*UHHFH A DC Distribution Network with Alternative Sources R. Magureanu, M. Albu, M. Priboianu and A.M. Dumitrescu University Politehnica Bucharest/Department of Electrical Engineering, Bucharest, Romania Abstract— A DC grid with different type of generators and load is presented. It is considered an islanded and grid connected operation. A low voltage laboratory model is simulated and tested and is expected in the future the results to be extended to high voltage applications. It is demonstrated that the solution can have not only scientific but also commercial viability. I. INTRODUCTION Romania has two main sources for renewable energy, The first one is hydro, which is producing around 30 % of total electrical energy and the second one is biomass, produced by the forests which are covering over 40 % of total country area. Less developed, but with a possible positive results are wind and solar energy produced by wind turbines and solar cells [1], [2], [3]. The disadvantage is the fact that generator’s current is no longer sinusoidal but has a high content of harmonics and the rectified voltage has relatively high ripple (Fig. 3). Either the rectification is done by a unity power factor converter, at a high cost and complicated control or it can be used a six-phase rectifier with 12 valves (or a twelvepulse system), supplied by a synchronous generator with two separate sets of three phase windings [4], resulting in a six phase system (Fig. 5). Fig. 2. Water turbine flow/speed/efficiency characteristic ia2/10 (Amps) 800 600 400 200 Fig. 1 Map of Romania II. -200 va2 (V) As all these types of energy are distributed and sometimes they are used only for local consume and other times are connected to the main grids, such a network has to present also storage facilities as the production of energy is variable from day to night and winter to summer. 0 -400 -600 -800 0.97 0.98 0.985 Time (s) 0.99 0.995 1 a) Voltage and current for three phase generator 900 DC SOLUTIONS FOR SYNCHRONOUS GENERATION OPERATION 750 vDC (V) The generation of electrical energy is typically done by synchronous or asynchronous generators for hydro and wind turbines and by power electronics for sun and electrochemical sources. Typical for hydro and wind generators is the fact that their efficiency is function of wind and water flow and converter rotation speed. Due to the fact that synchronous generator has to operate at constant speed; these conditions are difficult to be fulfilled (Fig. 2). Consequently, a possible solution is to rectify by diode bridges generator outputs and to parallel them in DC. 0.975 600 450 300 150 0 0.98 0.982 0.984 0.986 0.988 0.99 0.992 Time (s) 0.994 0.996 0.998 b) Rectified output voltage Fig. 3 Voltages and currents of the three-phase generator 1 7 3URFHHGLQJVRIWKHWK0HGLWHUUDQHDQ&RQIHUHQFHRQ &RQWURO$XWRPDWLRQ-XO\$WKHQV*UHHFH Other solution is to use the simple rectification plus a booster which to maintain the output voltage at a constant level. In Fig. 4 there is presented the voltage and the current for a load that is increasing from 50% of nominal power to 100 % of nominal power. Fig. 5 Six-phase generator-rectifier system a) Voltage and current with booster ia1/10 (Amps) 400 300 200 100 0 va1 (V) -100 -200 -300 -400 0.97 0.975 0.98 0.985 0.99 1 0.995 Time (s) a) b) The rectified voltage with booster Fig. 4 The voltages and currents with booster I. DC GRID STRUCTURE PROPOSED In the case in which a three phase with neutral distribution line (Fig. 7a) is available and is wanted to be transformed in a DC network, [9] [10] the system can be transformed in a two lines configuration. Using per unit values, the AC network can transport a power equal with 3, but the DC one only 2.44, which means a transmission efficiency of 0.81 (Fig. 7b). vdc2 (V) 900 750 600 vdc (V) vdc1 (V) 450 300 150 0 0.97 0.975 0.98 0.985 Time (s) 0.99 1 0.995 b) Rectified voltages Fig. 6 Voltages and currents of the six-phase generator R = 1Ω Uˆ = 1, 41U Uˆ = 2, 44U R = 1Ω I = 1A Uˆ = 1, 41U Uˆ = 2, 44 U R = 1Ω Uˆ = 1, 41U a) PCA = 3 The disadvantage is the fact that generator’s current is no longer sinusoidal but has a high content of harmonics and the rectified voltage has relatively high ripple. Either the rectification is done by a unity power factor converter, at a high cost and complicated control or it can be used a six-phase rectifier with 12 valves (or a twelvepulse system), supplied by a synchronous generator with two separate sets of three phase windings [4], resulting in a six phase system (Fig. 5). In order to reduce the harmonic level of the rectified output, we suggested [5] a solution similar to that employed in the case of three winding transformers: a configuration of the generator two winding systems in a star/wye connection, thus obtaining a six phase system [6], [7], [8]. In this way the harmonics are considerably reduced, the sixth harmonic of the rotating magnetic field in generator air gap is practically eliminated, improving not only the efficiency of the generator but also reducing the DC ripple of the DC output voltage and current (Fig. 6). Voltage and current for the first three phases 7 3URFHHGLQJVRIWKHWK0HGLWHUUDQHDQ&RQIHUHQFHRQ &RQWURO$XWRPDWLRQ-XO\$WKHQV*UHHFH R = 1, 73 Ω − Uˆ = 2 ⋅ 3 = 2, 44U + charged when there is an excess of energy in the system and to discharge then when energy is necessary. That means that we need a bidirectional DC/DC converter which to be controlled in such a way that this process to be optimally done (Fig. 8). Uˆ = 2U b) + RL I1 = 1A PCC = 2, 44 U CC = 2, 44 AC − PCC = 2, 44 U CC = 2, 44 AC + RL PCA = 4,88 − I1 = 1A c) Fig. 7 The three-phase line operating in AC (a), single DC (b) and double DC (c) Fortunately, we have available the extra phase and the neutral which can be used also doubling the power of the transport line, resulting 4.88 with a transport efficiency of 1.62 (Fig. 7c). As the current will be DC, the losses will be smaller, compared with the AC ones, and the current can be increased by at least 10% which is equivalent with total transport efficiency about 1.8. II. INTEGRATION OF ALTERNATIVE SOURCES IN A DC SYSTEM Romania is a country with a medium solar energy potential and although at present this is not a practical solution, taking into consideration that in the future, and the price of solar cells and of the energy produced will decrease, in the future it might be an interesting possibility, reason for each we introduced it in our experimental model, [11] [12]. Beside hydro, solar and wind synchronous generators, the system can use for electrical energy generation from biomass a fuel cell converter supplied through a biogas generator and a reformer. As the voltage produced by each cell is low (a little bit over 1 V) and is variable with the load, a unidirectional DC/DC converter is used in order that the output voltage to be controlled in such a way that this to be about equal with the DC line voltage [13]. There are different solutions for energy storage. We considered here only two as in our case it is expected that the energy available in the system from time to time is over the energy which can be consumed or delivered to the grid. The first solution is to produce hydrogen by the mean of a hydrolyser and, when electrical energy is necessary, that hydrogen to be used to produce energy. A second solution is to use led acid batteries which to be Fig. 8 Double DC distribution network As we have two parallel lines, an equalizing system was suggested, transferring the energy from one leg to the other by the mean of a bidirectional DC/DC converter. The connection with the grid is done by two bidirectional front-end converters which allows not only transforming the energy from DC in AC and DC in AC, but can also improve the quality of energy on AC side. In Fig. 9 is presented one of the two front-end converters of 60 kVA which can operate as presented (Fig. 10a, b). 7 3URFHHGLQJVRIWKHWK0HGLWHUUDQHDQ&RQIHUHQFHRQ &RQWURO$XWRPDWLRQ-XO\$WKHQV*UHHFH III. CONCLUSIONS The use of DC current in transport of energy improves efficiency of operation and the quality of power. The basic elements are the power electronic converters of different types, able to convert the AC and DC currents in a suitable way. In this paper, new grid architecture for integrating dispersed generation is proposed. Simulation results concerning the power electronic converters and their operation into the proposed system are presented. Further work will emphasize laboratory testing of a DC grid in kAmps range, as a first step towards a commercial exploitation (20 MW, 10 kV DC). ACKNOWLEDGMENT The authors are much obliged to the Ministry of Education and Research for financing the project trough three grants in the period of 2006-2008. 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